docs: def check_column_existence(col_name, df, presence=True):
      """
      Checks whether or not `col_name` is in `df` and raises a helpful error msg
      if the desired condition is not met.
  
      Parameters
      ----------
      col_name : str.
          Should represent a column whose presence in `df` is to be checked.
      df : pandas DataFrame.
          The dataframe that will be checked for the presence of `col_name`.
      presence : bool, optional.
          If True, then this function checks for the PRESENCE of `col_name` from
          `df`. If False, then this function checks for the ABSENCE of
          `col_name` in `df`. Default == True.
  
      Returns
      -------
      None.
      """
      if presence:
          if col_name not in df.columns:
              msg = "Ensure that `{}` is in `df.columns`."
              raise ValueError(msg.format(col_name))
      else:
          if col_name in df.columns:
              msg = "Ensure that `{}` is not in `df.columns`."
              raise ValueError(msg.format(col_name))
      return None